FIG. 1 is a cross-sectional view schematically showing a structure of a conventional electronic component 70 including a hollow or cavity sealing structure for a MEMS (Microelectromechanical System) device. The electronic component 70 includes a substrate or board 71 having a surface on which a functional unit, such as a SAW element including an interdigital transducer (IDT) electrode 72 and the like can be formed. A cavity 80 is formed on the substrate 71 to cover the functional unit. The lateral surface of the cavity 80 is defined by a metal layer 75 and the top surface thereof is defined by a foil 76. The metal layer 75 and the foil 76 are covered with a copper plating layer 77. The copper plating layer 77 is covered with a first resin layer 78. Solder bumps 79 are disposed on a surface of the electronic component and connected to the copper plating layer 77.
FIG. 2 is a cross-sectional view showing a package in which the electronic component 70 is mounted on a printed circuit board 86 and sealed with a second resin layer 89. The electronic component 70 is mounted face-down for welding the solder bump 79 to an electrode pad 87 disposed on the main surface 86a of the printed circuit board 86. After the welding, the solder bump 79 forms a solder layer 83. The electronic component 70 is then sealed with the second resin layer 89 by transfer molding or compression molding. The sealed and packaged electronic component 70 can be used as an electronic device.
The process of transfer molding or compression molding may apply an external pressure onto the electronic component 70 that may press the surface of the substrate 71. Accordingly, the copper plating layer 77 is formed in a shell structure to tolerate the external pressure and maintain the cavity 80. The copper plating layer 77 has a thickness of, for example, 30 μm to maintain the shell structure. The first resin layer 78 has a thickness of, for example, 30 μm and a gap having a thickness of, for example, 60 μm is provided to solder connect the electronic component 70 to the printed circuit board 86. The solder connection is applied between the solder layer 83 and the electrode pad 87.
Japanese Patent Application Publication No. JPH05-90885A discloses a technology for face-down mounting of a SAW element by which a functional surface of the SAW element is opposed to the circuit board on which it is mounted. According to this technology, the SAW element is connected and secured to a circuit board via an annular member formed of silicone resin or the like of a certain thickness to establish a space for the oscillation of the SAW element, and the element is then potted with resin. U.S. Pat. No. 6,417,026 discloses a technology by which a chip device is mounted face-down on a mounting substrate provided with bump electrodes and the chip device is sealed and protected with resin. A protection layer covering the space above an active region of the chip device and an insulating layer enclosing the active region are formed for preventing the resin from flowing into a gap between the chip device and the mounting substrate. The technologies disclosed in these references are directed to a resin potting process and do not include a process of transfer molding or compression molding that is commonly performed under high temperature and pressure.